The present invention generally relates to methods of growing group III-V compound semiconductor epitaxial layers, and more particularly to a method of growing a group III-V compound semiconductor epitaxial layer by use of an atomic layer epitaxy.
Presently, the use of an atomic layer epitaxy (hereinafter simply referred to as an ALE) is becoming popular when growing a compound semiconductor epitaxial layer. The ALE is sometimes also referred to as a molecular layer epitaxy (MLE). For example, when growing a group III-V compound semiconductor epitaxial layer by use of the ALE, a compound including group V atoms and a compound including group III atoms are alternately supplied to a substrate, so as to grow epitaxial layers in atomic monolayers.
The following features are obtainable by the ALE. Firstly, the ALE is virtually unaffected by the growing conditions, that is, the results of the epitaxial growth virtually do not change even when the growing conditions are changed. Secondly, the thickness of the epitaxial layers can be controlled in atomic monolayers. Thirdly, it is possible to grow an epitaxial layer having a satisfactory surface morphology. Fourthly, it is possible to grow a uniform epitaxial layer.
The above described features are obtainable by the ALE for the following reasons. That is, the epitaxial growth is carried out completely in two dimensions because a deposition of a monolayer of first atoms (or molecules) is followed by a deposition of a monolayer of second atoms (or molecules). In addition, the supplied atoms (or molecules) have a self-limiting effect at the surface of the substrate. Due to the self-limiting effect, atoms (or molecules) amounting to only one atomic layer (or molecular layer) are deposited per operation cycle, and additional atoms (or molecules) will not be deposited on the grown layer even when excess atoms (or molecules) are supplied.
FIGS. 1A through 1E show models for explaining a growth of a gallium arsenide (GaAs) layer on a (100) plane (Miller index (100)) of the substrate. In FIGS. 1A through 1E, .sym. denotes a gallium (Ga) atom and x denotes an arsenic (As) atom.
FIG. 1A shows a vicinity of the surface of a GaAs monocrystal substrate SUB. The substrate SUB is completed by forming the As last.
In order to grow a GaAs monocrystal layer on the substrate SUB by use of a metalorganic chemical vapor deposition (hereinafter simply referred to as an MOCVD), a quantity of Ga amounting to one layer in the GaAs monocrystal is supplied as shown in FIG. 1B. It will be assumed hereunder that the MOCVD is used for the epitaxial growth.
FIG. 1C shows a state where the supply of Ga is ended and one Ga layer is formed on the substrate SUB.
Next, as shown in FIG. 1D, As is supplied to grow one As layer on the Ga layer. Since As has the self-limiting effect, no excess As is deposited on the Ga layer. For this reason, there is no need to accurately control the supply quantity of As.
FIG. 1E shows a state where the supply of As is ended and one As layer is formed on the Ga layer. In FIG. 1E, L3 denotes one GaAs layer amounting to one molecular layer formed on the substrate SUB.
The processes of FIGS. 1B through 1E may be repeated when it is necessary to grow additional GaAs layers.
All of the features described before are obtained with the epitaxial layer which is grown by the ALE. However, even in the case of an epitaxial layer made of a material which does not have the self-limiting effect, it is possible to obtain the features obtainable by the ALE, except the first feature, by controlling the supply time of the material and controlling the growth rate per material supply cycle so as to grow the epitaxial layer in atomic monolayers.
As described heretofore, the ALE is an important technique of growing a compound semiconductor epitaxial layer. The present inventors have conducted various experiments related to the growth of group III-V compound semiconductor epitaxial layer by the ALE using trimethylaluminum (TMA: (CH.sub.3).sub.3 Al)/arsine (AsH.sub.3) as the source material. From the experiments, it was found that aluminum (Al) which is in most cases an essential constituent element of the group III-V compound semiconductor epitaxial layer does not have the self-limiting effect for one atomic monolayer.
Al is an essential material constituting a compound semiconductor device, and for example, a superlattice made up of stacked AlAs layer and GaAs layer is often used. In this case, it is desirable that a transition at an interface between the AlAs layer and the GaAs layer occurs suddenly. For example, a superlattice has an AlAs layer amounting to three atomic monolayers and a GaAs layer amounting to three atomic monolayers which are stacked. Al is also used in a compound semiconductor device which requires a fine composition profile control, such as a high electron mobility transistor (HEMT), resonant-tunneling hot electron transistor (RHET) and multiquantum well (MQW) semiconductor laser.
Therefore, in order to grow a group III-V compound semiconductor epitaxial layer having Al as the group III element, it is desirable to use the ALE. However, as described before, Al does not have the self-limiting effect for one atomic monolayer. For this reason, the supply time of the material (Al) must be controlled to control the growth rate per material supply cycle so as to grow in atomic monolayers.
But even when the TMA/AsH.sub.3 is used as the source material and the flow rate condition is set for the TMA so that an Al layer amounting to one molecular layer is grown in one material supply cycle, it was found from experiments that the Al epitaxial layer obtained does not have the features of the epitaxial layer grown by the ALE. Specifically, the surface morphology and uniformity of the Al epitaxial layer are poor.